I'm not particularly well-versed in RC setups, which brings me here for a little advice.

I am gutting an RC helicopter for a project. I need to lower the rotor speed significantly, among other things. The stock motor is a 440 size; can diameter of 28mm and height of 35 mm. I'd like any replacement motor to be close to that.

I need to lower the overall rotation speed to 100 RPM's or less. The gearing is around 10:1, so a motor output of 1000 RPM's or less is what I'm looking for.
What kind of brushless motor setup should I start looking into?

What kind of power will you be needing? The power requirement will determine how much current you will need at a given voltage, which may set a minimum voltage. That voltage will determine the kV of the motor. For example, if a 1 cell LiPo (3.7V) will provide the necessary power, then a 270kV motor will provide the rpm you're looking for. If you need 100W, finding a 1 cell LiPo that will provide 27 amps or more may be a problem, though. Going to 2 cell will give you a greater selection of batteries, but then you'd need a 135kV motor.

What kind of power will you be needing? The power requirement will determine how much current you will need at a given voltage, which may set a minimum voltage. That voltage will determine the kV of the motor. For example, if a 1 cell LiPo (3.7V) will provide the necessary power, then a 270kV motor will provide the rpm you're looking for. If you need 100W, finding a 1 cell LiPo that will provide 27 amps or more may be a problem, though. Going to 2 cell will give you a greater selection of batteries, but then you'd need a 135kV motor.

I don't know how much power I'll need, but providing the amperage for any given voltage should not be a problem because this thing does not need to move anywhere; the rotor only has to spin. Worse comes to worse I'll make a power supply or wire a couple of 3.7's in parallel.

I'd appreciate it if you could link me any motors with kv ratings that low; I've had trouble finding much below about 750 kv. I'm trying to get away with as little gearing as possible because lubricants and plastics typically don't fare well in a vacuum chamber.

I'm working with a group on a heliogyro spacecraft project. I'm going to skip past the explanation and say that it looks like the rotor on a helicopter, but the blades are much longer, thinner, and more flexible.

We're trying to get data for modeling in a vacuum chamber, which is where the helicopter comes in. It's got most of the hardware we need; it just spins too fast. When we finish, the whole thing will be stationary in the middle of the chamber receiving external power/control while the rotor+about .25 kg extra spins at 100 RPM or less. I'm trying to find a way to get there, preferably with minimum gearing, which is starting to look like a pipe dream.

Lossy. I'd go with a humongous ratio belt drive. Or even double reduction

Belts are no good, the allowable ratio is no better than gears. Double reduction is the only way you are going to get good efficiency If the efficiency is not so critical a worm gear would work well, just make sure there is a owb between the blades and the worm gear, or you may find that when you stop the motor the rotor inertia destroys your gears.

Belts are no good, the allowable ratio is no better than gears. Double reduction is the only way you are going to get good efficiency If the efficiency is not so critical a worm gear would work well, just make sure there is a owb between the blades and the worm gear, or you may find that when you stop the motor the rotor inertia destroys your gears.

Not quite true.

If the belt is constrained by idler gears or the motor is a long way from the output shaft any ratio is possible.. Its all about having enough belt on the smaller pulley.

How firmly are you locked into your motor size requirement? If you mounted a deep-ratio planetary gearbox to a low-Kv outrunner, you could probably get the shaft RPMs down pretty low to start with and then gear it even more with the heli's stock gearing. The only tricky part might be finding a pinion to fit the gearbox, as most on the market are made for directly driving a prop which results in a pretty large shaft diameter.

How firmly are you locked into your motor size requirement? If you mounted a deep-ratio planetary gearbox to a low-Kv outrunner, you could probably get the shaft RPMs down pretty low to start with and then gear it even more with the heli's stock gearing. The only tricky part might be finding a pinion to fit the gearbox, as most on the market are made for directly driving a prop which results in a pretty large shaft diameter.

It looks like I'll be making my own mounting however I do it, so motor size isn't much of a constraint anymore.